The present disclosure relates to an image sensor.
An image sensor is a semiconductor device that converts an optical image into an electrical signal, and is largely classified into a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
The CIS includes a photodiode region for receiving and converting a light signal into an electrical signal and a transistor region for processing the electrical signal, where the photodiode and the transistor region are placed horizontally.
In relation to the horizontal image sensor, the photodiode region and the transistor region are horizontally disposed on a semiconductor substrate so that there exists a limitation in expanding a light detecting portion in a limited area (this is typically referred to as a “Fill Factor”).
In order to overcome this limitation, as one approach, a photodiode is deposited using amorphous silicon (Si) above circuitry formed on a Si substrate. As another approach, circuitry is formed on a Si substrate and a photodiode is formed on a separate substrate and disposed on the Si substrate using a wafer-to-wafer bonding method. The photodiode is provided on and connected to a readout circuit of the circuitry. This configuration is referred to as a three-dimensional (3-D) image sensor. The photodiode and the circuitry are connected through a metal line.
However, in relation to the wafer-to-wafer bonding method, because the bonding surface of a wafer may not be even, its bonding strength may be deteriorated. The unevenness is often due to a metal line for connecting the photodiode with the circuitry being exposed to the surface of an interlayer insulation layer. Therefore, the interlayer insulation layer may have an  uneven surface profile. As a result, the interlayer insulation layer may have less bonding power with respect to the photodiode formed thereon.